A major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D plays an important role in maintaining skeletal calcium balance by promoting calcium absorption in the intestines, promoting bone resorption by increasing osteoclast number, maintaining calcium and phosphate levels for bone formation, and allowing proper functioning of parathyroid hormone to maintain serum calcium levels. Vitamin D exists in several different forms (vitamin D1 through D5); the major forms are c vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is produced photochemically in the skin in the presence of certain wavelengths of light. The precursor molecule, 7-dehydrocholesterol, is produced in relatively large quantities in human skin and when it reacts with UVB light (in the 270-300 nm wavelength range), and yields pre-vitamin D3, which spontaneously isomerizes to vitamin D3. The metabolism of vitamin D in humans involves two major metabolic steps. First, the liver readily modifies vitamin D to produce 25-hydroxyvitamin D, which has little activity. Subsequently, the kidney metabolizes 25-hydroxyvitamin D into the active hormone, 1, 25-dihydroxyvitamin D. Production of 1, 25-dihydroxyvitamin D is carefully regulated by the body according to mineral requirements.
It is understood that those who do not have enough exposure to sunlight with certain ultra violet wavelengths tend to suffer from vitamin D deficiency (hypo-vitaminosis D), due to their inability to synthesize vitamin D. This health issue arises since sunlight is required for vitamin D synthesis in humans.
While vitamin D has been used as a supplement for many decades, the supplement's oral bioavailability dynamics are not well understood. In addition, vitamin D is known to have impaired oral absorption in individuals with biliary or hepatic dysfunction or fat mal-absorption syndromes. Further, the generation of the active metabolites 25hydroxyvitamin D and 1,25dihydroxyvitamin D are impaired in renal and hepatic disorders.